The invention relates to the processing of the contrast density of an image in a radiography device. More particularly, the invention is directed to interpretation or reducing the appearance of artifacts for image reconstruction.
In numerous X-ray imaging applications where the object of which images are being taken has a very low contrast, a contrast agent is injected for the purpose of improving the contrast of the object. For example, a contrast agent of this sort is used for cardiology examinations or for mammographs or for DSA (Digital Subtraction Angiography) examinations. In many cases, the visibility of the object is further improved by acquiring an image called a mask image before injecting the contrast agent. This mask image is then subtracted from the image made after the contrast agent is injected in order to obtain a final image that will be used. In this case, a logarithmic subtraction is carried out. A logarithm of the mask image and the logarithm of the image made with the contrast medium. The logarithm of the mask image is subtracted from the logarithm of the image made with the contrast medium. The resulting image gives a good idea of the distribution of the concentration of the contrast agent but has drawbacks. Because of the beam hardening of the X-ray radiation, the contrast of an image made with a given density of contrast agents depends on the amount of tissue that is superimposed. In the case of DSA examinations, which require high concentrations of contrast agents, the beam hardening of the X-ray radiation may also be due to the contrast agent itself. This drawback is problematic when trying to interpret the images resulting from the subtraction and generally leads to artifacts when using the images for image reconstruction, in particular, three-dimensional construction.